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Pump Station Control: On/Off Control Strategy (Part 2)

By David W Spitzer and Walt Boyes

E-Zine June 2015

Click here to read “Pump Station Control: Introduction”
Click here to read “Pump Station Control: On/Off Control Strategy (Part 1)”

The level switch signals can also be generated from a signal from a level transmitter by using signal trip modules or software in the sequencer. The significant advantage of this approach is that it provides additional level measurement information beyond the presence or lack of level. In many cases, the level measurement can provide information that allows the diagnosis of a potential problem prior to the problem affecting the process. In addition, changing the level at which the pump turns on or off can be performed electronically instead of mechanically.

The calibration of the level transmitter should be checked occasionally to ensure proper operation. On the other hand, a level switch typically requires little maintenance until it fails. Note that failure of either a level transmitter or level switch can cause major problems. However, it is often possible to use the level measurement information to detect a problem prior to it affecting the process.

In some applications, one operating pump cannot handle the design load. In this case, additional level switches (or trips) can be used to sequence the operation additional pumps and alternate the pumps. For example, a three-pump station may be sequenced 1-2-3 or 1-3-2, and may operate based upon:

Level Operating Pumps
90% alarm
80% 3 pumps
70% 2 pumps
60% 1 pump
20% off

As the level rises, more pumps are turned on to handle the load, creating four flowing rates --- zero, one pump, two pumps and three pumps. Due to pumping and piping hydraulics, when at least one pump continues to operate, turning another pump on or off will not affect the flow rate as much as the single pump operation described above. Therefore, designing to keep at least one pump on continuously tends to reduce these abrupt flow rate changes.

Click here to read “Pump Station Control: Modulating Control Strategy (Part 1)”

From Flow Control (June 2002)

ISSN 1538-5280

Spitzer and Boyes, LLC
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